The property of isotropy in a material implies that all physical properties, including elastic, plastic, and strength properties, are invariant with respect to the direction of observation. Anisotropy, on the other hand, implies that the various physical properties of the material are a function of the direction of observation. For example, wood is an anisotropic material in that its properties in the direction of the grain are different from those in a direction perpendicular to the grain.
When a material which is initially isotropic is extruded, it will become anisotropic unless the microstructure of the material is allowed sufficient time to relax toward isotropy before the material is quenched.
During the extrusion process, the material is generally extended in the direction of extrusion and contracted in the directions perpendicular to the direction of extrusion. In the case of extrusion of sheets, the material is extended in the directions of extrusion and of the width of the sheet, but contracted in the direction of the thickness of the sheet. If the material is a long chain polymeric substance, the extrusion process will tend to align the molecules in the direction of the extrusion. Since the long chain polymeric chemical bonds are generally stronger than cross-linking bonds between chains, the strength of the aligned polymer is greater in the direction of alignment than perpendicular to it. In the case of a polycrystalline solid, the slip systems within the crystal grains will cause the deformed grains to align themselves in the direction of extrusion with material strength implications similar to that for a long chain polymeric material, i.e., greater tensile strengths in the direction of extrusion than transverse to the direction of extrusion.
For extrusions such as pipe, conduit, drain gutters, and other cross-sectional shapes, the tensile strength transverse to the direction of extrusion may be more important than that in the direction of extrusion. For example, in a tube or pipe under internal pressure, the wall stress in an axial direction is half that of the wall stress in a tangential direction. Yet, with conventional pipe extrusion, the strength of the material in the axial direction is greater than that in the tangential direction. Thus, in conventional extrusion, the material for piping or tubing is disadvantageously oriented. In like manner, for a conventionally extruded profile shape or sheet, the bending strength in the direction transverse to the direction of extrusion is less than that in the direction of extrusion, because the transverse elongation during extrusion is less than the elongation in the direction of extrusion.
There exists a method of orienting the extruded material somewhat in the tangential direction while it is already oriented in the axial direction (termed bi-axial orientation) by enlarging the diameter (and hence the circumference) of the tubing or pipe at the end of the extrusion process. (See,e.g., P.V.C. Technology, Fourth Ed., W. V. Titov, Elsevier Applied Science Publishers, 1984, Essex England, P. 882; The Encyclopedia of Plastics Equipment, pages 425-427, ed, H. R. Simonds, Reinhold Publishing Corp., New York, 1964). However, for a significant tangential elongation, the resulting tubing is limited in thickness. Similarly, the material of a sheet extrusion can be bi-axially oriented by stretching it in the transverse direction after it is extruded.
An interesting type of material orientation on a larger scale than heretofore discussed is described in Winton L. Slade, "Method and Apparatus for Extruding Polytetraflouroethylene Tubing" U.S. Pat. No. 3,008,187, Nov. 14, 1961. It was noted that in the manufacture of said tubing from a powder of the tubing material which was initially suspended in an organic extrusion aid to form a paste and then formed into an annular billet, when the billet was extruded through a tube forming die, the powder of the tubing material formed fibres which were oriented in the direction of extrusion. After extrusion the tubing was sintered and the fibrous structure and associated voids disappeared. However, the resulting tubing seemed to seep low viscosity fluids and thus Slade provided a remedy which consisted of having large helical grooves on the male and female portions of his extrusion die. Since the direction of extrusion inside the grooves was helical, the opposing helices of the male and female extrusion die members resulted in opposing helical fibre structures of the material on the inner and outer surfaces of the tubing which after sintering and resulting shrinkage, had an inhibiting effect on fissuring and resulting seepage. It is to be noted that in the Slade process, the initially powdered state has no orientation, inasmuch as the granules are of amorphous and isotropic material and may be considered as being approximately spherical in shape. The orientation of the extruded state is formed by the extrusion process and lies in the local direction of extrusion; virtually all of the granules in the grooves become aligned fibres in the direction of the grooves. For greatest effect, it is important for the maximum amount of granules to pass through the grooves and the minimum amount of granules to pass through the annulus between both sets of grooves. The recommended angle between both sets of grooves and hence fibres is from 15.degree. to 60.degree., and the recommended depth of each of both sets of grooves is from two to five times the thickness of the annulus between the grooves according to Slade. It is also to be noted that in extruding a long chain polymer or a polycrystalline solid, the initial orientation of the molecular chains or crystal grains is random and that after extrusion, there is a statistical directional orientation distribution that favors the direction of extrusion, but that also leaves some chains or grains still disadvantageously oriented. For a polycrystalline solid, the strengthening effect in the direction of elongation is known as strain-hardening.
Slade also discloses the possibility of rotating the male portion of the die relative to the female portion of the die during extrusion.
Another interesting process involving large strains superposed on extrusion or drawing of rods and pipes is describes in Sinnathamby Thiruvarudchelvan "Method and Apparatus for Forming Elongated Articles Having Reduced Diameter Cross Sections" U.S. Pat. No. 4,300,378, Nov. 17, 1981. This process consists of "a method and apparatus for forming elongated articles whereby a torque is transmitted to the deforming material as it passes through a die cavity to facilitate the reduction in cross-section as it passes therethrough." The torque twists the rod being extruded about the axis of symmetry by rotating at least one part of the die during operation. In the case of pipe extrusion, the rod is concomitantly pierced by a smooth mandrel. The stress in this case is a shearing stress on a plane perpendicular to the axis of symmetry and in a direction perpendicular to the radius from the axis. The deformation produced is similar to that in a pack of playing cards held between the palms of both hands with one palm then rotated in its plane relative to the other. This is not the effect operating in the present invention.